There are many, many differences between standing on the peak of a snow-capped mountain surveying the world below you, and lying in a plastic tent with a pump sucking the oxygen out of it. But for endurance athletes, the question that matters is: is the thin air you find on top of mountains equivalent to the air in an altitude tent for the purposes of stimulating physiological adaptations that will make you faster? The question turns out to be trickier than you might think, as some new studies make clear. We often think of altitude as simply having less oxygen, but that's not quite right. The air at high altitude has the same percentage of oxygen (roughly 21%) as everywhere else; it's just that there's less air overall, so the pressure is lower. That means you breathe in less oxygen with each breath. In laboratories and altitude chambers, we can achieve the same goal (less...